Noise reduction for an acoustic cooling system

ABSTRACT

The present invention relates to an acoustic cooling system ( 1 ) arranged for cooling by generating sound waves, said system ( 1 ) comprising a transducer ( 2 ) and a control unit ( 6 ) configured to generate a drive signal (S 1 ) for exciting said transducer ( 2 ), wherein said drive signal is a multi-harmonic drive signal comprising at least one higher harmonic (A 2 -A 5 ) selected to reduce the presence of at least one corresponding higher harmonic (B 2 -B 5 ) comprised in the sound waves. The system is advantageous in that noise-reduction can be achieved without the need of incorporating a second transducer, thereby enabling a compact acoustic cooling system at a low cost.

TECHNICAL FIELD

The present invention relates to an acoustic cooling system arranged for cooling by generating sound waves.

BACKGROUND OF THE INVENTION

Today techniques exist where a warm object is cooled by means of sound waves generated by an acoustic cooling system, an example being an acoustic-resonance system. The most essential component of such as system is an acoustic transducer, i.e. a piezoelectric element, PVDF (polyvinylidine difluoride) material, a loudspeaker or any other electrodynamic, electromagnetic, or electrostatic transducer. When the transducer is connected to a resonator, such as an open resonant pipe or a Helmholtz resonator, a pulsating airstream is generated. This airstream is used for cooling purposes in e.g. electronic circuits and systems or in luminaries. The pulsating airflow is more effective in cooling than the laminar airflow obtained when employing more conventional cooling techniques.

Although an acoustic cooling system can be made pretty silent if operated at frequencies that are low enough (e.g. lower than 70 Hz), higher harmonics may be generated by nonlinear behavior of the transducer and by nonlinear behavior of the pulsating airstream itself. These higher harmonics cause disturbing audible noise.

WO2008/053435 describes how noise generated by an acoustic cooling system can be countered by means of a second transducer driven in such a way that the audible noise generated by the first transducer is compensated. However, as this kind of sound cancellation requires a second transducer, it has a detrimental effect on cost. Thus, there is a need for an alternative solution that is able to reduce noise generated by the acoustic cooling system without requiring a second transducer.

SUMMARY OF THE INVENTION

In view of the above, an object of the invention is to at least alleviate the problem discussed above. According to an aspect of the invention, this and other objects are achieved by an acoustic cooling system arranged for cooling by generating sound waves. The system comprises a transducer and a control unit configured to generate a drive signal for exciting the transducer, wherein the drive signal is a multi-harmonic drive signal comprising at least one higher harmonic selected to reduce the presence of at least one corresponding higher harmonic comprised in the sound waves generated by the acoustic system.

The present invention is based on the understanding that by deliberately introducing at least one specifically selected higher harmonic into the drive signal which is used to excite the transducer, the presence of at least one corresponding higher harmonic in the sound waves generated by the acoustic system may be reduced, and thus a reduction in audible noise generated by the acoustic cooling system can be achieved. The inventive cooling system is advantageous in that noise-reduction can be achieved without the need of incorporating a second transducer. This enables a compact acoustic cooling system at a low cost.

A multi-harmonic signal is here intended to indicate a signal comprising a first harmonic (i.e. a main frequency) and one or more specifically selected higher harmonics. Further, the sound waves for the transducer may preferably be generated in a fluid, such as air. Furthermore, the transducer may comprise a resonator, which is advantageous in that a desired resonance may be obtained for a pulsating airflow.

The acoustic cooling system may further comprise a sensor, such as a microphone, adapted to detect sound waves generated by the system and to provide a sound signal related to the detected sound waves to the control unit, wherein the control unit is further configured to select the at least one higher harmonic of the drive signal based on the sound signal. An advantage is that the actual noise level (e.g. noise arising due to the complete system including noise arising due to non-linear behavior in the pulsating airstream) may be measured, and the drive signal may be adapted accordingly.

According to an alternative embodiment, the system may comprise a sensor adapted to measure at least one of an induced voltage and an induced current in the transducer as an indication of the sound waves generated by the system and to provide information related to at least one of the induced voltage and the induced current to the control unit, wherein the control unit is further configured to select the at least one higher harmonic of the drive signal based on the information. This may eliminate the need of a microphone.

According to another aspect of the invention, there is provided a method for determining a multi-harmonic drive signal for an acoustic cooling system arranged for cooling by generating sound waves. The method comprising the steps of providing a drive signal to a transducer, acquiring a sound signal relating to sound waves generated by the system in response to the drive signal, determining a sound power spectrum for the sound signal, and transforming the drive signal by introducing at least one higher harmonic selected such that at least one corresponding higher harmonic in the sound power spectrum is reduced.

At least one of an amplitude and a phase for the at least one higher harmonic in the drive signal may be selected such that a corresponding higher harmonic in the sound power spectrum is reduced. For example, the amplitude and/or phase of the second harmonic in the drive signal may be adapted to minimize the second harmonic in the sound power spectrum, the amplitude and/or phase of the third harmonic in the drive signal may be adapted to minimize the third harmonic in the sound power spectrum, etc. An advantage is that the noise associated with each higher harmonic can be minimized.

According to an embodiment, the multi-harmonic drive signal may be determined by a procedure that iteratively introduces increasingly higher harmonics to the drive signal, starting with the second harmonic. Furthermore, the acoustic cooling system according to the present invention may advantageously be included in an illumination device further comprising a light emitting device, wherein the acoustic cooling system may be arranged for cooling the light emitting device.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

FIG. 1 schematically illustrates an acoustic cooling system according to an embodiment of the invention;

FIG. 2 schematically illustrates a flow chart for an embodiment of a method for determining a multi-harmonic drive signal; and

FIG. 3 schematically an exemplary illumination device comprising an acoustic cooling system.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Referring now to the drawings and to FIG. 1 in particular, there is depicted an acoustic cooling system 1 comprising a transducer 2. The transducer 2 may be a piezoelectric element, PVDF (polyvinylidine difluoride) material, a loudspeaker or any other electrodynamic, electromagnetic, or electrostatic transducer. The transducer 2 shown in FIG. 1 is connected to a resonator 3 with an opening 4. The opening 4 is typically directed towards a warm object, such as a light emitting diode 5 (LED) that is to be cooled during operation.

The cooling system 1 further comprises a control unit 6 arranged to generate a drive signal 51 for exciting the transducer 2, such that sound waves in the form of a pulsating airstream are generated at the opening 4 for cooling the LED 5. The control unit 6 may be connected to a sensor, such as a microphone 7, arranged at a predefined distance from the cooling system 1 and arranged to detect sound waves (or noise) generated by the system 1. Here, the control unit 6 comprises a generator element 8 (such as a signal generator) for generating a signal with a basic frequency, a transformation element 9 (such as a signal processing unit) for transforming the signal from the generator element 8, and an analyzing element 10 for determining a sound power spectrum of a sound signal S_(sound) acquired by the microphone 7.

The control unit 6 may include a microprocessor, a microcontroller, a programmable digital signal processor or another programmable device. The control unit 6 may also, or instead, include an application specific integrated circuit (ASIC), a programmable gate array programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit 6 includes a programmable device such as the microprocessor or microcontroller mentioned above, the processor may further include computer executable code that controls operation of the programmable device. Additionally, portions of the functionality provided by the control unit 6 may be realized by means of analogue electronics.

Alternatively of detecting sound waves generated by the system by means of the microphone 7, it may be possible to use a voltage or current detector 11 to detect the current through the transducer 2 or the voltage over the transducer 2 as an indication of the sound waves (or noise) generated by the system 1. For example, a relationship between the sound waves generated by the system 1 and the induced voltage/current may be known from prior testing. Thus, the induced voltage/current acquired by the voltage/current detector 11 may be transformed into a sound signal that may be sent to the analyzing element 10 in the control unit.

According to the invention, a multi-harmonic drive signal may be used to reduce the higher harmonics of the sound waves generated by the system 1. In addition to a main frequency component (also referred to as first harmonic), the multi-harmonic drive signal includes one or more higher harmonics.

An embodiment of a method for determining a multi-harmonic drive signal will now be described with reference to the system in FIG. 1 and the schematic flow chart in FIG. 2. This procedure may be performed during assembly of the cooling system 1 by using a temporary microphone 7. Alternatively, the microphone 7 may be included in the acoustic cooling system to enable subsequent calibration, or continuous adaptation of the multi-harmonic drive signal.

During an exemplary operation, the signal generator 8 generates a single harmonic signal, such as a sinusoidal signal with a frequency of e.g. 60 Hz. Initially, the signal passes the transformation element 9 unchanged and thus the transducer 2 is excited, in step 201, by a single harmonic drive signal S1 which only has a first harmonic A1. The sound waves generated by the system 1 in response to the single harmonic drive signal S1 is detected by the microphone 7, and a sound signal S_(sound) related to the detected sound waves is sent to the analyzing element 10 in the control unit 6 in step 202. The analyzing element 10 determines a sound power spectrum for the acquired sound signal in step 203. The sound power spectrum of the sound signal here has a first B1, second B2, third B3, fourth B4, and fifth B5 harmonic.

Then the transformation element 9 transforms the drive signal by introducing, in step 204, a second harmonic A2 to the drive signal S1. As the transducer is excited with the transformed signal S1 the sound power level of the second harmonic in the sound power spectrum is reduced. Furthermore, the amplitude A and/or phase φ of the second harmonic may be adjusted, in step 205, such that the sound power level of the second harmonic is minimized (i.e. the second harmonic of the sound waves generated by the system is minimized). This can be done by starting with a phase (φ=0 and/or a small amplitude and gradually increasing the phase and/or amplitude (while monitoring the sound power spectrum that results as the transducer is excited by the transformed drive signal) until a minimum sound power level is found for the second harmonic in the sound power spectrum.

The procedure described in relation to step 204 and 205 may then be repeated for increasingly higher harmonics (i.e. for the third harmonic, and then for the fourth harmonic, and so on) until the sound level has been minimized for all higher harmonics present in the sound power spectrum.

As the transducer 2 is excited with the resulting multi-harmonic drive signal (which includes a main frequency A1 and higher harmonics A2-A5), only the first harmonic B1 remains in the sound waves generated by the system. That is, all higher harmonics have been, at least partly, reduced and consequently the noise generated by the system 1 has been reduced.

Although the procedure of determining the multi-harmonic drive signal has here been described as an iterative procedure, where increasingly higher harmonics are introduced one at the time, it may be possible to introduce and optimize the parameters for a plurality of higher harmonics in parallel to speed up the procedure. Moreover, other noise cancellation algorithms, or control algorithms in general are applicable to determine the optimum settings for each higher harmonic in the drive signal. For example, Iterative Learning Control may be used.

Turning now to FIG. 3, schematically illustrates an embodiment where the acoustic cooling system 1 is included in an illumination device, such as a lamp 12, further comprises a light emitting device, such as am LED 5. The procedure of determining the multi-harmonic drive signal for the acoustic cooling system may be varied depending on the application. As an example, the single multi-harmonic drive signal may be determined for the acoustic cooling system during development of the lamp, and then this predetermined multi-harmonic drive signal may be applied for many lamps. For example, the same multi-harmonic drive signal may be utilized for all lamps of a specific model, thus reducing cost. Alternatively, the multi-harmonic drive signal for the acoustic cooling system may be determined for each individual lamp during manufacturing. This may be advantageous if the spread between different lamps is too large to justify a single multi-harmonic drive signal.

As another example, the multi-harmonic drive signal may be tuned during operation. This may be advantages if the lamp 12 behavior in terms of higher harmonics appears to vary a lot over time or depending on how the lamp 12 is mounted. This could be done as an initial calibration, or the calibration could be repeated occasionally. It may also be possible to use continuous adaptation of the multi-harmonic drive signal.

Additionally, the microphone may not only be used to detect the sound output of the lamp, but other neighboring lamps as well. This enables use of a different frequency or phase of the drive signal for various neighboring lamps such that the overall sound output of all lamps will be decreased, thereby avoiding sub-optimization of one lamp only.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example, to account for the spread in different lamps it may be possible to apply the optimization procedure to a set of lamps. This may be achieved by using various uncoupled multi system systems, e.g. one acoustic cooling system per lamp, or one acoustic cooling system for a cluster of lamps, and another acoustic cooling system for another cluster of lamps. Moreover, instead of using a microphone for each acoustic cooling system, a plurality of acoustic cooling systems may share a single microphone. Yet another alternative would be to have a plurality of microphones share a single control unit. Additionally, although the acoustic cooling system has here been described for cooling of an LED, the system may also be used for cooling of other electronic components such as, for example, integrated circuits, or microprocessors. Furthermore, in the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. 

1. An acoustic cooling system arranged for cooling by generating sound waves, said system comprising: a transducer; and a control unit configured to generate a drive signal for exciting said transducer, wherein said drive signal is a multi-harmonic drive signal comprising at least one higher harmonic selected to reduce the presence of at least one corresponding higher harmonic comprised in the sound waves.
 2. Acoustic cooling system according to claim 1, further comprising a sensor configured to detect sound waves generated by the system and to provide a sound signal (S_(sound)) related to the detected sound waves to said control unit, wherein the control unit is further configured to select the at least one higher harmonic of the drive signal based on said sound signal (S_(sound)).
 3. Acoustic cooling system according to claim 1, further comprising a sensor configured to measure at least one of an induced voltage and an induced current in the transducer as an indication of the sound waves generated by the system and to provide information related to at least one of the induced voltage and the induced current to said control unit, wherein the control unit is further configured to select the at least one higher harmonic of the drive signal based on said information.
 4. A method for determining a multi-harmonic drive signal for an acoustic cooling system arranged for cooling by generating sound waves, the method comprising the steps of: providing a drive signal to a transducer; acquiring a sound signal (S_(sound)) relating to sound waves generated by the system in response to the drive signal; determining a sound power spectrum for said sound signal (S_(sound)); and transforming said drive signal by introducing at least one higher harmonic selected such that at least one corresponding higher harmonic in the sound power spectrum is reduced.
 5. A method according to claim 4, wherein at least one of an amplitude and a phase for said at least one higher harmonic in the drive signal is selected such that a corresponding higher harmonic in the sound power spectrum is reduced.
 6. A method according to claim 4, wherein the multi-harmonic drive signal is determined by a procedure that iteratively introduces increasingly higher harmonics to the drive signal, starting with the second harmonic.
 7. An illumination device comprising: a light emitting device; and an acoustic cooling system according to claim 1 arranged for cooling said light emitting device. 